I'd expect that the only type of place that would retro fit sensors and camera same as factory would be a proper retro fitter of VW Group bits and it will cost serious money.
Even if you took this job on yourself, it would cost serious money just for the parts, extra programming is also needed, so you would either need to buy VCDS etc or get a useful VAG Indie to do that for you.
Getting a SEAT dealer to retro fit parking sensors will normally only mean getting a Vodaphone kit fitted that will not integrate into your car's systems fully, but in practical terms these parking sensors will a lot better than nothing.
I did an "as factory" retro fit of front parking sensors into my S4, very expensive but in my mind worth it.
No VW Group dealership will accept the work of retro fitting stuff as per factory, as firstly, they will not have anyone that can do that work quickly/well/right first time, and secondly, their hourly rate would mean that they would get get your business.